7. A trachoid time crystal: Any machine has lots of moving parts, and the TARDIS is no different. One of those parts, mentioned in the "Doctor Who" serial "The Hand of Fear," from 1976, is a trachoid time crystal.
Oh, come on! Does such a thing even exist? Well, according to Nobel Prize-winning physicist Frank Wilczek, something like it just might be possible - mathematically, at least. He was teaching a class on 3-D crystals and started wondering whether you could have a crystal-like structure in the fourth dimension of time - i.e., a time crystal. A couple of scientific papers later, he concluded that, indeed, you could.
As Alexandra Witze explains in Science News:
To visualize a time crystal, think of Earth looping back to its same location in space every 365¼ days; the planet repeats itself periodically as it moves through time. But a true time crystal is made not of a planet but of an object in its lowest energy state, like an electron stripped of all possible energy.
Witze goes on to compare the "time crystal" to a perpetual motion machine that doesn't defy the laws of thermodynamics, because it's already in its lowest energy state; hence, no work can be extracted from the system. If Wilczek has a found a way to beat the Second Law of Thermodynamics, we should just go ahead and make him the next Doctor. I'm sure Matt Smith wouldn't mind.